1. Field of the Invention
This invention relates to plasticized polyvinyl chloride resins having improved flame retardancy and, in particular, it relates to methods for producing highly flame retarded plasticized polyvinyl chloride resins containing the di-2-ethylhexyl ester of tetrabromophthalic acid and, optionally, a flame retardant enhancer such as antimony oxide.
2. Description of the Prior Art
Plasticized polyvinyl chloride (PVC) resins are a well known class of thermoplastic resins which combine excellent chemical and corrosion resistance, good physical strength, and excellent electrical insulation properties. These properties may be extensively modified through the choice and concentration of the plasticizer and other formulation components. The terms "plasticized polyvinyl chloride," "polyvinyl chloride," and "PVC" are used herein to refer broadly to these classes of resins.
Because unplasticized PVC contains chlorine in the polymer structure, the polymer has an inherent degree of flame retardancy. The more rigid types of PVC, containing a minimal concentration of plasticizer, usually require only additional antimony trioxide to impart a satisfactory level of flame retardancy. However, as the amount of plasticizer is increased to yield more flexible resins, the degree of flammability significantly increases.
Both triaryl and diaryl alkyl phosphate esters have been used for some time to increase the flame retardancy of PVC resins. When used in modest addition to the primary plasticizer, these esters provide adequate retardancy in many low-risk applications. However, as continually higher flame retardancy demands are made of the resins to meet more stringent applications, increased concentration of these esters does not yield the desired retardancy level. Another group of materials, halogenated paraffins, also provide flame retardancy but are subject to limited use due to their relatively poor thermal stability.
U.S. Pat. No. 4,298,517 and its division, U.S. Pat. No. 4,397,977, each describe and claim halogenated resins such as PVC incorporating polyoxyalkylene tetrahalophthalates. Example 26 of the patents also describes plasticizing PVC with a mixture of 28 parts dioctyl phthalate and 12 parts of dioctyl tetrabromophalate. However, these patents do not recognize the exceptional thermal stability and high flame retarding ability of the 2-ethylhexyl tetrabromophthalate agent of this invention.
Therefore, it is the principal object of this invention to provide plasticized polyvinyl chloride compositions having a higher than usual level of flame retardancy while acceptably maintaining other important properties.